This application focuses on achieving a better understanding of the relationship between the microstructural properties and the biomechanical response of dentin. The premise is that by focusing on dentin as an engineering material, the clinical design of restorations can be optimized and the possibility of catastrophic failure minimized. The applicant seeks to establish the mechanisms for the anisotropic, elastic and inelastic, age and time dependent response of dentin, and to develop and verify a material model for dentin structure. Because of the small of the specimens that can be fabricated from human dentin, the applicant and his collaborators are developing a novel experimental technique to observe the properties of miniature dentin specimens 1.0 x 1.0 x 1.5 mm with precise tubule orientations within the specimens. The strain field under compression loading is measured optically from an array of sputter-coated grid lines on the specimen surface. High- definition video recording with computerized image processing will be employed to obtain the strain data. Micro-hardness testing will be conducted and correlated to the observed mechanical properties so that mechanical properties can be inferred from micro-hardness measurements in later experiments and in other laboratories. The model to be developed will include both position along the tubule (pulp to DEJ or CEJ) and age as variables. The hypotheses are: 1) in a given tooth, the elastic moduli and proportional limit are a function of position along the tubule, and the elastic properties are the same at similar positions in a tooth, and 2) the elastic moduli and proportional limit are a function of the amount of intertubular mineralization, which increase with age. These hypotheses are tested in five specific aims: 1) develop a correlation between the elastic properties and the microstructure of dentin, taking age-related changes into [account], 2) establish a correlation between the mechanical properties of dentin and the response of dentin as measured in micro-hardness testing, 3) establish a working hypothesis that describes the physical mechanisms for the inelastic, time-dependent (viscoelastic) response of dentin, 4) develop a physically based constitutive equation for the anisotropic, age-dependent elastic response of dentin, and 5) implement the constitutive model in a finite element code to simulate the age- dependent, elastic response of a simple dentin structure that can be reliably evaluated in an experiment. This application is for support of the University of Cincinnati portion only of a joint research project with the School of Dental Science of the University of Melbourne. The two universities have a twenty-year history of engineering collaborations. The principal investigator has only two years experience with the University of Melbourne School at Dental Science and lists no joint publications with the Australian dental collaborators (Drs. Messer, Wilson, Palamara, or Thomas).